Kōzaburō Arase serves as a detective within the Third Materials Compiling Group (Comps-3) of the Shinjuku Police's community safety division, having come from the Organized Crime Countermeasures Division, Section 4 (Anti-Gang Unit). His history includes membership in the yakuza before joining the police, though his reasons for leaving that life remain undisclosed. He maintains ties to his underworld connections, leveraging them for unpublicized illegal cover operations that ultimately triggered disciplinary action from superiors like Danjo Tozawa. Standing 176 cm tall, Arase possesses short black hair, brown skin, and a lean, athletic frame. A large yakuza tattoo depicting a tiger superimposed over flowers and flowing water covers his back, sometimes revealed unintentionally during duty. His standard attire consists of a black shirt and trousers, sometimes paired with a grey jacket. Arase exhibits a dual nature: outwardly calm and polite with a slight smile during routine interactions, he explodes into fierce violence when provoked. Colleagues liken him to a "wild animal" or "carnivore," observing his instinctive proficiency in violence and pursuit. He becomes most animated during physical confrontations, unleashing sudden, overwhelming force that starkly contrasts his usual quietness. Known as The Quiet One in Comps-3, he typically positions himself as a stoic shadow beside his partner Tsubaki Iwanome, allowing her to handle dialogue until his intervention becomes necessary. Despite his police role, Arase openly acknowledges his own reliance on violence, self-identifying as a "scumbag." He holds a low opinion of criminals and "lowlifes," reflecting his disdain for his past affiliations. This was evident in his brutal dismantling of the gang Sons of the Styx, where he assaulted eight members near to death, an act that dissolved the gang and left a lasting impression on leader Kuon Higuro and informant Takumi Kuruya. In operations, Arase responds to threats with abrupt violence, such as head-butting thugs demanding money in a stairwell. He later intercepted a drone piloted by Takumi Kuruya, deducing the hacker's involvement and warning associates Corpse God and Misaki Sakimiya to sever ties due to Takumi's criminal history. This encounter ended with Misaki kicking Arase to escape, leaving a visible shoe imprint on his chest. Forensic analysis by colleague Saki Aikawa later matched this imprint to a footprint from the "Human Knot" case. During Comps-3 briefings, Arase displays disinterest in procedural discussions, often doodling a skeleton instead of focusing on cases like the Shakuzawa Building skeletons or Human Knot incidents. His vigilance occasionally lapses, as when he failed to notice the infiltration of the mercenary Lemmings into the department.